Surgical emulsification is a technique for removing diseased tissue which utilizes an ultrasonic handpiece attached to a hollow needle which, using ultrasonic energy, emulsifies and removes the diseased tissue. The equipment used for emulsification also includes a pump, various valves and tubing lines which remove fluid and emulsified tissue pieces by aspiration through the handpiece.
The phacoemulsification technique for cataract removal (described in Phacoemulsification Surgery, Devine and Banko, Eds., Pergamon Press, New York, (1991)) is generally practiced as follows. A hollow needle, acoustically coupled to an ultrasonic handpiece and surrounded by a hollow sleeve, is inserted into the anterior chamber of the eye through a small (2-3 mm) incision in the cornea, and the tip of the needle is brought into contact with the cataract tissue. The handpiece includes an ultrasonic transducer which may be either piezoelectric or magnetostrictive. When the handpiece is activated, the needle is vibrated longitudinally at an ultrasonic rate. Simultaneously, a hydrodynamic flow of saline solution is introduced into the eye in order to prevent collapse of the anterior chamber. The vibrating needle emulsifies the nucleus of the cataract, and the particles are simultaneously aspirated, along with fluid, out of the eye through the hollow phacoemulsification needle. Preferably, the hard cataract material is emulsified within the thin transparent capsule surrounding the lens of the eye in order to avoid undesirable injury to healthy ocular tissues in the corneal endothelium and iris. Aspiration is effected by the vacuum pump, which is connected to the handpiece. Because particles of cataract tissue have an abrasive character and may damage the walls of the anterior chamber, it is important that they be removed as quickly and as completely as possible during the phacoemulsification procedure.
The ultrasonically vibrated needle emulsifies the cataract by the combined effect of at least four different forces, namely (i) the mechanical impact of the needle tip which varies depending on its mass, sharpness, and acceleration; (ii) the ultrasonic acoustical wave generated by the metal surfaces of the vibrating needle; (iii) the fluid wave created at the needle's leading edge and (iv) implosion of cavitation bubbles created at the tip of the vibrating needle. Cavitation bubbles are micron-sized and expand and implode within a few acoustic cycles, thereby creating forceful shock and fluid waves. It has been observed that a conventional phacoemulsification needle produces millions of 80-150 micron cavitation bubbles at its tip during operation.
Prior to the present invention, attempts were made to improve the efficiency of emulsification. For example, U.S. Pat. No. 5,213,569 discloses a phacoemulsification needle having focusing surfaces for concentrating the acoustical energy such that the resulting focal point of acoustical energy is located outside the needle. Such designs are intended to capitalize on the tissue emulsifying effect of cavitation bubbles, but have the disadvantage of amplifying the fluid wave, thereby opposing the evacuation of fluid and waste and having the undesired effect of pushing tissue debris away from the tip. This, in turn, slows the emulsification process and subjects the patient to prolonged surgical manipulation. Moreover, cavitation bubbles occurring outside the needle may damage healthy tissues.